Conventional polyurethane wheels are manufactured by coating a steel hub with an adhesive layer, placing the hub in a mold, and pouring liquid polyurethane into the mold where the adhesive layer bonds the polyurethane to the steel hub. The adhesive layer may include an epoxy, phenolic, or urethane layer, for example. When manufacturing a wheel in this manner, the polyurethane and adhesive layer are typically cured together at a specific temperature. For proper curing to occur, the wheel assembly must reach a uniform temperature. Large wheels, for example up to 300 lbs. (136 kg) or more, are especially difficult to cure because of their large thermal inertia.
However, the adhesive-enabled bond between the polyurethane tire and the hub fails at an unacceptable rate. Moreover, manufacturers cannot reliably determine whether a particular bonded wheel, or batch of bonded wheels, will fail because the quality of the bond cannot be verified during manufacture.
Aside from manufacturing issues, dissipation of heat during use of wheels is also of concern. Bonds between a hub and a wheel fail because of a property called hysteresis. Hysteresis occurs during heavy loads on the wheel whereby the polyurethane tire heats up and melts near the hub, causing the polyurethane tire to slip, or tear, off of the hub. To mitigate hysteresis, larger forklifts sometimes use tires with grooves where loads and operating conditions are extreme. The purpose of the grooves is to release heat in order to prevent tire failure due to hysteresis. To mitigate the problem of hysteresis with polyurethane tires, other materials, such as rubber, have been used.
Using rubber tires may mitigate the hysteresis problem, but rubber tires are an unsatisfactory substitute to polyurethane tires in many applications, such as with lift trucks. In such applications, polyurethane tires do not wear as fast as rubber tires, have a higher weight capacity than rubber tires, have reduced rolling resistance resulting in prolonged battery life, and exhibit superior abrasion and chemical resistance as opposed to rubber tires. Moreover, as a result of the higher weight capacity, most electric lift trucks require polyurethane tires because of the additional weight of the batteries used to power the lift and propel the lift truck. Therefore, for many types of lift trucks, and other applications, polyurethane tires may be required.
Another disadvantage of polyurethane tires is that they are made of a harder compound than rubber tires, making an operator's ride more uncomfortable, particularly on uneven ground. Moreover, in such conventional polyurethane wheels, a user replaces a polyurethane tire by replacing the entire polyurethane tire/metal hub combination as a result of the bond between the tire and hub thereby wasting the hub where only the polyurethane tire is worn.
Therefore, there is a need for an improved means for affixing polyurethane tires to a hub so that the polyurethane tire may be replaced while allowing the metal hub to be reused. Moreover, there is a need for improved manufacturing processes for producing polyurethane wheels.